The need exists in a large number of fields to perform grinding operations on workpieces. For example, a billet is often the raw material for a rolling process in which the billet is flattened and formed into a finished product. These billets often contain surface imperfections which, if not removed, are carried through to the finished product. Accordingly, these imperfections are normally removed in a grinding process called "spotting," in which a rotating grinding wheel is held against the surface imperfection until the surface imperfection is removed. The surfaces of billets are also normally coated with a layer of oxides and other material. This surface layer must also be removed in order to prevent the surface layer from degrading the quality of the finished product. The surface layer is normally removed in a process called "skinning," in which the billet reciprocates beneath the grinding wheel while the grinding wheel is held in contact with the billet. After each grinding pass, the grinding wheel is stepped or "indexed" transversely across the surface of the workpiece that is being skinned.
The skinning process is often performed automatically. In an automatic skinning operation, the grinding pressure of the wheel against the workpiece is automatically controlled while the workpiece reciprocates back and forth adjacent the grinding wheel, and the grinding wheel indexes across the workpiece an incremental distance each grinding pass.
During any grinding application, whether automatic or manual, skinning or spotting, the position of the grinding wheel is normally controlled by two hydraulic actuators of conventional design. Generally, one actuator primarily controls the force of the grinding wheel against the workpiece, while the other actuator "indexes" or moves the grinding wheel transversely across the workpiece an incremental distance each grinding pass. However, the indexing actuator may have some effect on the grinding force and the grinding force actuator may affect the transverse position of the grinding wheel.
A problem encountered in the use of hydraulic actuators to index the grinding wheel is the relatively slow response speed of the actuators. The actuators normally consist of a cylinder divided into two chambers by a piston. A rod extending from the piston is coupled to the grinding wheel to move the grinding wheel as fluid flows into and out of the chambers of the cylinder. Fluid flow into and out of the actuator is normally controlled by a servo valve having an electrical control input. The signal applied to the control input is normally derived from a comparison of the actual transverse position of the grinding wheel to the desired position of the grinding wheel. As the desired position of the grinding wheel rapidly changes, the signal applied to the control input of the servo valve changes accordingly. However, the response time of the control loop is generally slower than desired. The speed at which the automatic skinning procedure can be conducted is severely limited by a relatively slow indexing system, thus limiting the throughput of such conditioner grinders. The operating speed of the system can be increased only by increasing the loop gain of the system, but this has a tendency to make the system unstable.
In order to increase the response speed of servo control systems in which the feedback signal is derived solely from a position signal, feedback signals indicative of the first derivative of the position with respect to time (i.e., the velocity) have been used. Feedback servo systems utilizing velocity or rate feedback result in "damping" of the system, thus allowing the gain of the position loop to be increased above what would otherwise be permissible. As a result, the response speed of the servo system is increased, However, even servo systems utilizing velocity feedback are fairly slow. Furthermore, systems utilizing velocity feedback are highly susceptible to instability produced by excessive phase shift in the servo system. A phase shift 180.degree. is not uncommon, thus causing negative velocity feedback to become positive feedback and result in instability. Thus, although velocity feedback may improve the performance of some position feedback servo systems, its advantages are nevertheless limited in general applications for indexing control systems.